B replies:a. He looks like Peter Pan would look if he gained twenty pounds
b. He looks like Peter Pan if he gained twenty pounds.
c. He looks like Peter Pan has gained twenty pounds
d. He looks like Peter Pan who has gained twenty pounds.

Which of the above sentences are grammatically correct?
Which are natural?

Presumably the person has a face like Peter Pan and is probably wearing clothes like Peter Pan's but is a lot heavier.